(AP Photo/Kirsty Wigglesworth) FILE - In this Jan. 31, 2010 file photo, an unmanned U.S. Predator drone flies over Kandahar Air Field, southern Afghanistan, on a moon-lit night. The White House has no intentions to end CIA drone strikes against militant targets on Pakistani soil, setting the two countries up for diplomatic blows after Pakistani's parliament unanimously approved new guidelines for the country in its troubled relationship with the US, US and Pakistani officials say. (AP Photo/Kirsty Wigglesworth, File)

WASHINGTON (Reuters) - The U.S. government has authorized the killing of American citizens as part of its controversial drone campaign against al Qaeda even without intelligence that such Americans are actively plotting to attack a U.S. target, according to a Justice Department memo.

The unclassified memo, first obtained by NBC News, argues that drone strikes are justified under American law if a targeted U.S. citizen had "recently" been involved in "activities" posing a possible threat and provided that there is no evidence suggesting the individual "renounced or abandoned" such activities.

The document was disclosed as a bipartisan group of U.S. senators called on the Obama administration to release to Congress "any and all" legal opinions laying out the government's understanding of what legal powers the president has to deliberately kill American citizens.

The senators who signed the letter, including members of the Senate Intelligence Committee, said the administration's cooperation would "help avoid an unnecessary confrontation that could affect the Senate's consideration of nominees for national security purposes."

Obama has nominated John Brennan, his White House counterterrorism adviser, who defends drone strikes, to lead the CIA.

An Intelligence Committee confirmation hearing on the nomination is scheduled for Thursday, and Brennan is likely to face questioning on drone policy.

One national security official said the leak of the Justice Department memo may have been timed to blunt such congressional demands for the release of additional, possibly classified, documents relating to the U.S. use of drones.

Senator Dianne Feinstein, the Democrat who chairs the Senate Intelligence Committee, in a statement on Tuesday said she had been calling on the administration to release legal analyses related to the use of drones for more than a year.

Feinstein said the document published by NBC had been provided to congressional committees last June on a confidential basis, and that her committee is seeking additional documents, which are believed to remain classified.

In the unclassified Justice Department paper posted by NBC on its website, the authors laid out three conditions that the executive branch should meet before a drone strike is ordered.

'IMMINENT THREAT'

A top U.S. official must determine that the targeted person "poses an imminent threat of violent attack against the United States," cannot be captured, and that the strike "would be conducted in a manner consistent with applicable law of war principles," the department said.

A Justice Department spokeswoman declined to comment to Reuters about the report.

The memo is drawing new attention to the 2011 strike that killed Anwar al-Awlaki, a U.S.-born alleged leader of Al Qaeda's Yemen-based affiliate who U.S. investigators linked to a botched plot to blow up a U.S. airliner with a bomb hidden in a man's underwear on Christmas Day, 2009.

Targeted killings, carried out by remotely piloted unmanned aircraft, are controversial because of the risks to nearby civilians and because of their increasing use. The United Nations recently launched an investigation into their use.

Most such attacks have been carried out by the United States, but Britain and Israel have also used drones.

Hina Shamsi, The head of the American Civil Liberties Union's National Security Project, which has sued for more information on the drone program, called the memo "profoundly disturbing" and "a stunning overreach of executive authority."

The ACLU's Hina Shamsi in a statement called on the Obama administration to release what she said was a 50-page classified legal document on which the 16-page summary is based.

"Among other things, we need to know if the limits the executive purports to impose on its killing authority are as loosely defined as in this summary, because if they are, they ultimately mean little," she said late Monday.

The ACLU on Tuesday will also file court papers seeking to block government efforts to dismiss the group's lawsuit challenging the 2011 killing of Awlaki and two other Americans in Yemen, the statement said.

(Reporting by Susan Heavey, David Ingram and Mark Hosenball Editing by Warren Strobel and Vicki Allen)